The following apology was printed in the Guardian's Corrections and clarifications column, Tuesday March 17 2008

We were wrong to state that Michael Ancram MP claimed thousands of pounds to treat lichen and moss on a stately home from the House of Commons allowance scheme, when only £400 was spent on moss removal. We apologise for this error and accept that his allowance claim was made in relation to his constituency home in accordance with the scheme rules

Like dirty water from a dishcloth, accusations of political sleaze dripped from the weekend papers. Caroline Flint, the former housing minister, was shown to have splashed out on her own house. Derek Conway, a scandalous Tory backbencher, was in trouble again. And Jacqui Smith went on the BBC to justify her audacious expenses as "the right thing for the taxpayer". Among them all, however, one story stood out - the news that the Electoral Commission is to investigate donations to the Conservative party from its biggest corporate funder. The other stories, at worst, involve petty personal enrichment. Lord Ashcroft's involvement with the Tories goes to the heart of the link between money and power in British politics.

It is likely that the Electoral Commission will find no law was broken. Lord Ashcroft is a British citizen. Bearwood Corporate Services, the company through which he has channelled donations of more than £4m in the last six years, is registered in this country. It is also, the Conservative party insists, actively trading - and as such meets the requirements of the law. Labour MPs have been eager to do down Lord Ashcroft for years. It would be strange if he and his party had not made sure that they were acting within the rules.

But what the law allows and what is right for politics is not quite the same thing. Lord Ashcroft's position inside the Conservative party would be open to question even if it were shown beyond doubt that his donations were permissible and that he had, as he promised on taking his peerage in 2000, become a British taxpayer. He is, after all, much more than a generous donor. He is an activist with a tremendous power inside the Conservative organisation linked to the money he gives.

Conservatives react in two ways when Lord Ashcroft's name is mentioned. Some roll their eyes to the heavens. Others point out that he stayed loyal when the Tory cause seemed lost. Without his money and ideas, there might be no movement for David Cameron to lead today. What everyone agrees is that he is mysteriously exotic: his Labour opponents attribute incredible cunning to the big man from Belize. And no one seems to know what he expects in return.

David Cameron has come a long way by offering change. He argues that the old ways of doing political business must end. It may be legal but it is not healthy for an opposition party - let alone a government - to be so greatly in the debt of one man's generosity. The next time Bearwood Corporate Services offers to foot the bill, Mr Cameron would be very wise to refuse.